To date, attempts have been made to develop a number of therapeutic vaccines directed to tumor cells. This is because it is considered that there are qualitative or quantitative differences between tumor cells and normal cells that may be recognized by the immune system of a living organism, and the immune system stimulated by active and specific sensitization by vaccines utilizing such differences (neoepitopes) can recognize and eliminate tumor cells.
To bring about such anti-tumor response, at least two conditions may have to be met. Firstly, the tumor cells must express an antigen that does not appear in normal cells, or express an antigen to such an extent that normal cells and tumor cells can be distinguished solely in a qualitative manner. Secondly, the immune system must be activated by vaccines or such in order to react with the antigen of interest. A major obstacle in tumor immunotherapy is considered to be that the immunogenicity of tumors is particularly weak in humans.
In recent years, tumor-related and tumor-specific antigens including such neoepitopes that may constitute targets to be attacked by the immune system have been discovered. Nonetheless, the immune system cannot eliminate tumors expressing such neoepitopes, and this may be due to insufficient immune response to these neoepitopes, rather than due to the absence of neoepitopes.
Two general strategies have been developed for the purpose of cell-based cancer immunotherapies. One of them is adoptive immunotherapy where tumor-reactive T lymphocytes expanded in vitro are reintroduced into a patient, and the other is active immunotherapy which uses tumor cells to induce systemic tumor response by triggering new or stronger immune response to a tumor antigen.
Tumor vaccines based on active immunotherapy have been prepared by various methods. To induce immune response to a tumor antigen, irradiated tumor cells mixed with an immune-stimulating adjuvant such as Bacillus Calmette Guerin (BCG) (Non-Patent Document 1), tumor cells genetically modified to produce, for example, cytokines (Non-Patent Document 2), and alienated autologous tumor cells (Non-Patent Document 3) have been prepared. However, the immunogenicity of the tumor cells is low, and this is considered to be due to the quantity of the tumor antigen, not the quality.
On the other hand, antibodies are known to induce humoral immune responses (production of antibodies against an antigen) and cellular immune responses (production of CD8-positive T cells against an antigen) to antigens by cross-presenting bound antigens to antigen-presenting cells, and it has been reported that administration of an antibody can induce acquired immunity to an antigen (Non-Patent Document 4). Recently, for the anti-tumor effect by an anti-HER2 antibody, it has been shown in an in vivo mouse model that acquired immunity to the HER2 antigen induced by administration of the antibody plays a more important role than the direct ADCC of the administered antibody (Non-Patent Document 5). In fact, in clinical use of Herceptin, which is an IgG1 subclass antibody drug against HER2, acquired immunity was induced by Herceptin administration, and humoral immune response to HER2 was observed (Non-Patent Document 6). Since patients in whom Herceptin administration was effective particularly showed an increased anti-HER2 antibody titer, induction of acquired immunity by Herceptin administration was considered to play an important role in the anti-tumor effect.
Antibodies are highly stable in blood and have few side effects, and are therefore drawing attention as pharmaceuticals (Non-Patent Documents 7 and 8). Many studies have been carried out so far on antibody-dependent cellular cytotoxicity (hereinafter denoted as ADCC) and complement-dependent cytotoxicity (hereinafter denoted as CDC), which are effector functions of IgG class antibodies. It has been reported that in the human IgG class, antibodies of the IgG1 subclass have the highest ADCC activity and CDC activity (Non-Patent Document 9). Furthermore, antibody-dependent cell-mediated phagocytosis (ADCP), which is phagocytosis of target cells mediated by IgG class antibodies, is also suggested to be one of the antibody effector functions (Non-Patent Documents 10 and 11). Since IgG1 subclass antibodies can exert these effector functions against tumors, IgG1 subclass antibodies are used for most antibody pharmaceuticals against cancer antigens.
In order for IgG antibodies to mediate ADCC and ADCP activities, the Fc region of the IgG antibodies must bind to antibody receptors (hereinafter denoted as FcγR) that are present on the surface of effector cells such as killer cells, natural killer cells, and activated macrophages. In humans, isoforms FcγRIa, FcγRIIa, FcγRIIb, FcγRIIIa, and FcγRIIIb have been reported as members of the FcγR protein family, and their respective allotypes have been reported as well (Non-Patent Document 12).
Enhancement of cytotoxic effector functions such as ADCC and ADCP has been drawing attention as a promising means for enhancing the antitumor effects of anticancer antibodies. Importance of FcγR-mediated effector functions aimed for antitumor effects of antibodies has been reported using mouse models (Non-Patent Documents 13 and 14). Furthermore, it was observed that clinical effects in humans correlated with the high-affinity polymorphic allotype (V158) and the low-affinity polymorphic allotype (F158) of FcγRIIIa (Non-Patent Document 15). These reports suggest that antibodies with an Fc region optimized for binding to a specific FcγR mediates stronger effector functions, and thereby exert more effective antitumor effects. The balance between the affinity of antibodies against the activating receptors including FcγRIa, FcγRIIa, FcγRIIIa, and FcγRIIIb, and the inhibitory receptors including FcγRIIb is an important factor in optimizing antibody effector functions. Enhancing the affinity to activating receptors may give antibodies a property to mediate stronger effector functions (Non-Patent Document 16), and therefore has been reported in various reports to date as an antibody engineering technique for improving or enhancing the antitumor activity of antibody pharmaceuticals against cancer antigens.
Regarding binding between the Fc region and FcγR, several amino acid residues in the antibody hinge region and the CH2 domain, and a sugar chain added to Asn at position 297 (EU numbering) bound to the CH2 domain have been shown as being important (Non-Patent Documents 9, 17, and 18). Focusing on this binding site, studies have so far been carried out on mutants of the Fc region having various FcγR binding properties, and Fc region mutants with higher affinity to activating FcγR have been obtained (Patent Documents 1 and 2). For example, Lazar et al. have succeeded in increasing the binding of human IgG1 to human FcγRIIIa (V158) by approximately 370 fold by substituting Ser at position 239, Ala at position 330, and Ile at position 332 (EU numbering) of human IgG1 with Asn, Leu, and Glu, respectively (Non-Patent Document 19 and Patent Document 2). The ratio of binding to FcγRIIIa and FcγRIIb (A/I ratio) for this mutant was approximately 9-fold that of the wild type. Furthermore, Shinkawa et al. have succeeded in increasing the binding to FcγRIIIa up to approximately 100 fold by removing fucose from the sugar chain added to Asn at position 297 (EU numbering) (Non-Patent Document 20). These methods can greatly improve the ADCC activity of human IgG1 compared to that of naturally-occurring human IgG1.
While there are many reports, as described above, on methods for enhancing ADCC by antibody engineering, no reports have been made to date on antibody engineering techniques for enhancing or improving induction of acquired immunity by antibody administration. There is a report on methods for inducing acquired immunity against a cancer antigen, in which a cancer antigen against which acquired immunity is desired to be induced is fused with an antibody that binds to a high-mannose receptor or DEC-205 expressed on antigen presenting cells, thereby promoting incorporation and presentation of the cancer antigen by antigen presenting cells (Non-Patent Document 21). However, in these methods the target of antibody binding is not a cancer antigen as in the case of the above-mentioned anti-HER2 antibody. That is, since these methods induce acquired immunity against a cancer antigen fused to the antibody itself, the antibody itself cannot bind to the cancer antigen, and has the disadvantage of not being able to exhibit direct action on the cancer antigen. Furthermore, since this method induces acquired immunity not only against the cancer antigen fused to the antibody but also against the antibody itself used for targeting antigen-presenting cells, anti-drug antibodies will emerge and this leads to weakening of the effects. Therefore, this method may not be preferable for therapeutic purposes.
According to the above, while it is desirable to induce acquired immunity to a target antigen by administering an antigen-binding molecule having binding activity to the target antigen, there has been no reports on engineering techniques for improving or enhancing acquired immunity by such methods.